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The primary atomization is the basis of the following spray and mixture formation. High optical density as well as high spray velocities up to 350 m/s (50 MPa GDI) in combination with spray structures in the μm-range are great challenges for measurement techniques. Therefore, the processes of the primary atomization are not fully understood yet, although these are very important for simulations of spray and mixture formation in modern combustion engines. For gasoline sprays, the velocity field close to the nozzle outlet is an important information for understanding and modelling the spray breakup process. Common measurement techniques like Phase-Doppler-Anemometry (PDA) are only able to determine reliable data starting from a distance of 30 mm to the nozzle and low-pressure conditions. More specialized measurement techniques like x-ray Phase Contrast Velocimetry (PCV) used at the Argonne National Laboratory deliver velocities very close to the nozzle outlet, but are highly complex and allow only a very limited measurement time. For these reasons, the Structural Image Velocimetry (SIV) was developed to get a technique able to measure velocity fields in the first millimetres after the nozzle outlet for high-pressure fuel injections with an easier applicability. The SIV combines basic ideas of Laser Correlation Velocimetry (LCV) with the post-processing of Particle Image Velocimetry (PIV) to measure 2D velocity fields. Using a homogenous background illumination, structures inside the spray are obtained by a lens system with a very small focal depth and a high resolution (∼ 1 μm/px). Based on cross‑correlation, the structures are tracked and evaluated to determine a 2D velocity field. Comparisons with x-ray PCV measurements could be used to validate the SIV results. In this study, spray velocity measurements of the first millimetres of a modern gasoline injector are presented. The velocity fields on different operating conditions also including flash boiling (0.03 MPa gas pressure / 363 K fuel temperature) and high injection pressures up to 50 MPa are investigated.
